Polymer electrolyte membrane fuel cells (PEMFC) are sustainable, energy efficient, clean and environmental friendly, advanced power systems (Nature, 414 (2001), p. 345. Because of their potential to reduce our dependence on fossil fuels and diminish poisonous emissions to the atmosphere, PEMFCs have emerged as tantalizing alternatives to combustion engines. Using pure hydrogen which appears to be an excellent energy carrier and can be produced from any source by using solar, hydro, biomass, wind, geothermal, etc., PEMFCs are one of the most promising energy technologies under development, producing only water during their operation.
In a PEMFC, the proton exchange membrane (PEM) is the core part of the cell. The crucial properties of a membrane for proton exchange membrane fuel cell (PEMFC) are high proton conductivity, low electronic conductivity, good mechanical properties, oxidative and chemical integrity, low permeability to gases and limited swelling in the presence of water. It is obvious that is imperative the low cost and the capability for fabrication into MEAs.
Membranes commonly used in PEMFC are perfluorinated polymers containing sulfonic acid groups on side chains, like for example Nafion (U.S. Pat. No. 3,692,569), manufactured by DuPont. Such membranes suffer from high cost, low operating temperature and high fuel gas permeability. As a result, they exhibit good performance only at moderate temperatures (less than 90° C.), relative high humidities, and with pure hydrogen gas as fuel. The operation of a fuel cell at elevated temperatures has the benefits of reducing CO poisoning of the platinum electrocatalyst and increased reaction kinetics. That is why so much effort is presently devoted to the development of alternative PEM membranes able to stand higher temperatures. In this respect, new polymeric materials based on aromatic backbones have recently been synthesized in order to replace Nafion. Potential polymers for this aim include polyethersulfones, polyetherketones, polyimides and polybenzimidazoles.
Polymer electrolyte membranes that can be applied in fuel cells operating at temperatures above 120° C. are mostly based in polybenzimidazole (PBI) (U.S. Pat. No. 5,525,436). However, PBI presents moderate mechanical properties and low oxidative stability. These drawbacks in combination with the limited availability and it's high cost result in development of alternative polymeric materials. Various attempts have been made to improve the mechanical properties of PBI by using polymer blends composed of PBI and a thermoplastic elastomer (Macromolecules 2000, 33, 7609, WO Patent 01/18894 A2) in order to combine the acid doping ability of the PBI with the exceptional mechanical properties of the thermoplastic elastomer. Additionally, blends of PBI with aromatic polyether copolymer containing pyridine units in the main chain have also been prepared, resulting in easily doped membranes with excellent mechanical properties and superior oxidative stability (Journal of the Membrane Science 2003, 252, 115). Furthermore, scientific effort has been devoted to the development of alternative low cost polymeric systems that will combine all the desired properties for application in fuel cells operating at temperatures above 120° C.
This embodiment describes the prior art of making membrane electrode assemblies as specific to the membrane system described herein. The prior art covers issues described in literature covering the following areas: (i) direct membrane catalyzation, (ii) catalyzation of coated electrode substrates, (iii) need for effecting membrane electrode bonding for seamless proton transport (iv) effective solubility of reactant gases (in particular oxygen), (v) use of pore forming agents for effective gas transport within the electrode structure. This is with the specific objective of enhancing mass transport and the ability to operate a fuel cell on a sustained higher power density level.
In the context of these prior art as collated below it is our contention that our claims as enumerated in this application provide for a more effective control of interfacial transport of dissolved reactants, protons, and electrons while preventing and minimizing the dissolution of ionic component i.e., phosphoric acid or its improved analog under the broad classification of perfluorinated sulfonic acids (PFSA).
In the context of prior art, direct catalyzation of the membrane has been described in various patents and scientific literature primarily on aqueous based polymer electrolytes, most notably of the perfluorinated sulfonic acid type. At the current state of the technology, prior efforts together with current approaches have to be tempered with ability to translate developments in this regard to mass manufacturability keeping reproducibility (batch vs. continuous) and cost in perspective. Depending on the deposition methods used, the approach towards lowering noble metal loading can be classified into four broad categories, (i) thin film formation with carbon supported electrocatalysts, (ii) pulse electrodeposition of noble metals (Pt and Pt alloys), (iii) sputter deposition (iv) pulse laser deposition and (v) ion-beam deposition. While the principal aim in all these efforts is to improve the charge transfer efficiency at the interface, it is important to note that while some of these approaches provide for a better interfacial contact allowing for efficient movement of ions, electrons and dissolved reactants in the reaction zone, others additionally effect modification of the electrocatalyst surface (such as those rendered via sputtering, electrodeposition or other deposition methods).
In the first of the four broad categories using the ‘thin film’ approach in conjunction with conventional carbon supported electrocatalysts, several variations have been reported, these include (a) the so called ‘decal’ approach where the electrocatalyst layer is cast on a PTFE blank and then decaled on to the membrane (Wilson and Gottesfeld 1992; Chun, Kim et al. 1998). Alternatively an ‘ink’ comprising of Nafion® solution, water, glycerol and electrocatalyst is coated directly on to the membrane (in the Na+ form) (Wilson and Gottesfeld 1992). These catalyst coated membranes are subsequently dried (under vacuum, 160° C.) and ion exchanged to the H+ form (Wilson and Gottesfeld 1992). Modifications to this approach have been reported with variations to choice of solvents and heat treatment (Qi and Kaufman 2003; Xiong and Manthiram 2005) as well as choice of carbon supports with different microstructure (Uchida, Fukuoka et al. 1998). Other variations to the ‘thin film’ approach have also been reported such as those using variations in ionomer blends (Figueroa 2005), ink formulations (Yamafuku, Totsuka et al. 2004), spraying techniques (Mosdale, Wakizoe et al. 1994; Kumar and Parthasarathy 1998), pore forming agents (Shao, Yi et al. 2000), and various ion exchange processes (Tsumura, Hitomi et al. 2003). At its core this approach relies on extending the reaction zone further into the electrode structure away from the membrane, thereby providing for a more three dimensional zone for charge transfer. Most of the variations reported above thereby enable improved transport of ions, electrons and dissolved reactant and products in this ‘reaction layer’ motivated by need to improve electrocatalyst utilization. These attempts in conjunction with use of Pt alloy electrocatalysts have formed the bulk of the current state of the art in the PEM fuel cell technology. Among the limitations of this approach are problems with controlling the Pt particle size (with loading on carbon in excess of 40%), uniformity of deposition in large scale production and cost (due to several complex processes and/or steps involved).
An alternative method for enabling higher electrocatalyst utilization has been attempted with pulse electrodeposition. Taylor et al., (Taylor, Anderson et al. 1992) one of the first to report this approach used pulse electrodeposition with Pt salt solutions which relied on their diffusion through thin Nafion® films on carbon support enabling electrodeposition in regions of ionic and electronic contact on the electrode surface. See a recent review on this method by Taylor et al., describing various approaches to pulse electrodeposition of catalytic metals (Taylor and Inman 2000). In principal this methodology is similar to the ‘thin film’ approach described above, albeit with a more efficient electrocatalyst utilization, since the deposition of electrocatalysts theoretically happen at the most efficient contact zones for ionic and electronic pathways. Improvements to this approach have been reported such as by Antoine and Durand (Antoine and Durand 2001) and by Popov et al., (Popov 2004). Developments in the pulse algorithms and cell design have enabled narrow particle size range (2-4 nm) with high efficiency factors and mass activities for oxygen reduction. Though attractive, there are concerns on the scalability of this method for mass scale manufacturing.
Sputter deposition of metals on carbon gas diffusion media is another alternative approach. Here however interfacial reaction zone is more in the front surface of the electrode at the interface with the membrane. The original approach in this case was to put a layer of sputter deposit on top of a regular Pt/C containing conventional gas diffusion electrode. Such an approach (Mukerjee, Srinivasan et al. 1993) exhibited a boost in performance by moving part of the interfacial reaction zone in the immediate vicinity of the membrane. Recently, Hirano et al. (Hirano, Kim et al. 1997) reported promising results with thin layer of sputter deposited Pt on wet proofed non catalyzed gas diffusion electrode (equivalent to 0.01 mgPt/cm2) with similar results as compared to a conventional Pt/C (0.4 mgPt/cm2) electrode obtained commercially. Later Cha and Lee (Cha and Lee 1999), have used an approach with multiple sputtered layers (5 nm layers) of Pt interspersed with Nafion®-carbon-isopropanol ink, (total loading equivalent of 0.043 mgPt/cm2) exhibiting equivalent performance to conventional commercial electrodes with 0.4 mgPt/cm2. Huag et al. (Haug 2002) studied the effect of substrate on the sputtered electrodes. Further, O'Hare et al., on a study of the sputter layer thickness has reported best results with a 10 nm thick layer. Further, significant advancements have been made with sputter deposition as applied to direct methanol fuel cells (DMFC) by Witham et al. (Witham, Chun et al. 2000; Witham, Valdez et al. 2001), wherein several fold enhancements in DMFC performance was reported compared to electrodes containing unsupported PtRu catalyst. Catalyst utilization of 2300 mW/mg at a current density of 260 to 380 mA/cm2 was reported (Witham, Chun et al. 2000; Witham, Valdez et al. 2001). While the sputtering technique provides for a cheap direct deposition method, the principal drawback is the durability. In most cases the deposition has relatively poor adherence to the substrate and under variable conditions of load and temperature, there is a greater probability of dissolution and sintering of the deposits.
An alternative method dealing direct deposition was recently reported using pulsed laser deposition (Cunningham, Irissou et al. 2003). Excellent performance was reported with loading of 0.017 mgPt/cm2 in a PEMFC, however this was only with the anode electrodes, no cathode application has been reported to date.
However, in all these new direct deposition methodologies, mass manufacturability with adequate control on reproducibility remains questionable at best. In this regard the methodologies developed by 3 M company is noteworthy, where mass manufacture of electrodes with low noble metal 1 is reported (Debe, Pham et al. 1999; Debe, Poirier et al. 1999). Here a series of vacuum deposition steps are involved with adequate selection of solvents and carbon blacks resulting in nanostructured noble metal containing carbon fibrils which are embedded into the ionomer-membrane interface (Debe, Haugen et al. 1999; Debe, Larson et al. 1999).
An alternative is the use of ion-beam techniques, where the benefits of low energy ion bombardment concurrent to thin film vacuum deposition (electron beam) process is exploited for achieving dense, adhering and robust depositions (Hirvonen 2004). This method has been recently reviewed (Hirvonen 2004) in terms of both mechanisms of ion/solid interactions during thin film growth as well as development of various protocols for specific application areas, including tribology, anti corrosion coatings, superconducting buffer layers and coatings on temperature sensitive substrates such as polymers. Modifications of this approach to prepare 3-D structures including overhang and hollow structures have also been recently reported (Hoshino, Watanabe et al. 2003). Use of dual anode ion source for high current ion beam applications has also been reported recently (Kotov 2004), where benefits for mass production environment is discussed.
In this embodiment we describe a method for improving the catalyst utilization at the interface of a polymer electrolyte imbibed with ion conducting components (such as phosphoric, polyphosphoric and analogs of perfluorinated sulfonic acids) so as to enable higher power densities (i.e., 400 mW/cm2 at 0.5 V vs. RHE, 170-180° C., H2/Air). It is further stated that this improved power density is attained with lower Pt loading (0.3 to 0.4 mg/cm2) as compared to the current state of the art which is in the range 0.5 to 1.0 mg/cm2, thus providing for a better gravimetric energy density. A further manifestation of this embodiment is the improved ability to retain ion conducting elements (such as phosphoric, polyphosphoric and analogs of perfluorinated sulfonic acids) within the reaction layer (catalyst containing zone at the interface between the electrode and the membrane). This is particularly important from the perspective of long term sustained power density as well as better tolerance to both load and thermal cycling (especially transitions to below the condensation zone).